The tyrosine phosphorylation of specific protein substrates is an integral component of signal transduction pathways that mediate cellular differentiation and activation pathways. The pattern of expression of certain members of the src family of protein tyrosine kinases (and, in some cases, their association with surface receptors) suggested that these kinases are involved in these signal transduction pathways in hematopoietic cells. This hypothesis has been strengthened by recent functional studies that support the role of two src-related kinases, lck and fyn, in T cell activation. The hck gene is expressed in myeloid cells of both monocyte/macrophage and granulocyte lineages and is most highly expressed in mature cells of both lineages. The long term goal of this proposed research is to define the role of the hck protein in macrophage activation and myeloid differentiation. The primary strategy of this work is to manipulate the expression of hck or "activated" or "dead" mutants of the kinase in myeloid cells in order to explore the role of hck in activation and differentiation pathways. Preliminary data from this laboratory suggest that hck expression is not essential for granulocytic differentiation but do not exclude an important role of the kinase in mature granulocyte function. Preliminary studies in the murine macrophage line, Bac 1.2F5, show a strong correlation between hck expression and the activation of these cells to produce cytokines such as tumor necrosis factor (TNF). We hypothesize that hck expression is necessary though not sufficient for normal monocyte/macrophage differentiation and that hck plays an important role in the activation pathways of mature macrophages that lead to cytokine gene expression. Thus, the specific aims of this proposal are (1) To determine the role of hck in macrophage activation and the regulation of cytokine gene expression; (2) To determine the function of hck in monocyte/macrophage differentiation; and (3) To study the regulation of hck production, tyrosine kinase activity and subcellular localization in myeloid cells. These studies should significantly improve our understanding of the role of tyrosine kinases as critical components of the signal transduction pathways in hematopoietic cells.